The present invention relates to a current control method and associated device for an electric motor, and more particularly to a method and device for protecting a drive circuit of an electric power assist steering motor from over-current conditions.
Power assist steering systems are generally well known. Power assist steering systems assist a vehicle driver in steering a vehicle. More particularly, power assist steering systems assist a driver in steering the steerable wheels of a vehicle when the driver applies torque to the steering wheel of the vehicle.
Power assist steering systems are of many different types. Some known power assist steering systems provide assist force via the use of hydraulic power. Other power assist steering systems provide assist force via the use of electric power. An electric power assist steering system includes an electric motor that is drivably interconnected to steering components of the steerable wheels of the vehicle.
As with the operation of most electric motors, an electric motor used in an electric power assist steering system is energized or activated by current provided through stator windings of the electric motor. The current provided through the stator windings causes a rotor of the motor to rotate. The direction of the current through the stator windings controls the rotation direction of the rotor. Generally speaking, the magnitude of the current controls the torque provided by the motor.
Variable reluctance motors are commonly used in electric power assist steering systems because of their small size, low friction, and high torque to inertia ratio. In variable reluctance motors, typically separate stator windings are energized in pairs. More particularly, electric current is provided to the stator coils associated with a pair of stator poles. The rotor moves to minimize the reluctance between the energized stator poles and the rotor poles. Once a minimum reluctance is reached and the rotor poles thereby align with the energized stator poles, the energized stator poles are de-energized and an adjacent pair of stator poles are energized. The rotor then moves to minimize the reluctance between the newly energized stator poles and the rotor poles. The rotor is thereby caused to rotate based upon the sequence within which the stator poles are energized and de-energized.
A drive circuit is typically used to control the current that is provided to the motor to energize the stator windings. The drive circuit essentially regulates the energy delivered to the motor from a power source, such as a vehicle battery. Many types of drive circuits can be used to control the electric current provided to the motor. Needless to say, regardless of the type of drive circuit used, different electronic components are incorporated within the drive circuit to enable the circuit to accomplish its intended purpose. It is, generally well known that electronic components have operating tolerances, such as maximum temperature and current thresholds. Operating the components above these tolerances can affect the performance of the components and thereby potentially affect the operation of the drive circuit.
Some effort has been made to control the operation of motors so that operating conditions are maintained within tolerances. For instance, because it is generally well known that drive circuits and the components contained therein heat-up as current is provided to the motor, efforts have been made to control the amount of current provided to the motor. Heat sinks are often integrated into the drive circuits to control the temperature of the circuits. Also, a thermistor is commonly included in the system to provide thermal feedback of the temperature of the circuit. The feedback is used to reduce the power delivered to the motor when heat generated in the circuit causes temperatures to reach threshold levels.
However, most thermistors have a response time delay associated with their use. As a result, the temperature may exceed a threshold level by the time the threshold temperature is detected by the thermistor. Alternatively, because heat may continue to propagate throughout the circuit for a period of time after current is no longer provided to the motor, the temperature may continue to increase after a threshold temperature is sensed by the thermistor and the current is subsequently reduced. Moreover, once a threshold temperature is sensed by the thermistor and the current has been reduced, the latent heat of the circuit may not allow it to cool down in a sufficiently short amount of time. Essentially, because it takes some period of time for heat to dissipate, the components may be affected by the latent heat of the circuit.
U.S. Pat. No. 4,660,671 discloses an electronic control system for an electric motor coupled to a steering system. The patent discloses a drive circuit that includes four field-effect transistors connected in an H pattern, to effect switching within the motor. These as well as other components of the circuit control the energization of the electric motor. A fold-back circuit monitors the magnitude of current drawn by the motor and the temperature of the driving circuit. The fold-back circuit limits the current provided to the motor to prevent excess current from being applied to the motor. As the temperature of a heat sink increases, the maximum current permitted through the motor decreases.
Similarly, U.S. Pat. No. 5,475,289 discloses a method and apparatus for controlling an electric assist steering system. A motor current sensor is operatively connected to the electric motor for sensing the amount of current through the motor. The output of the current sensor is connected to a current fold-back circuit. If the sensed current through the motor is too high, i.e., greater than a predetermined value, the value of the torque demand signal is decreased to prevent switches from burning out.
According to one aspect, the present invention provides a method for protecting a drive circuit of an electric power assist steering motor. A charge accumulation from current provided to the motor is determined. The current provided to the motor is controlled based upon the determined charge accumulation.
According to another aspect, the present invention provides an over-current protection device for protecting a drive circuit of an electric power assist steering motor. Means determines a charge accumulation from current provided to the motor. Means controls the current provided to the motor based upon the determined charge accumulation.
According to another aspect, the present invention provides an over-current protection device for protecting a drive circuit of an electric power assist steering motor. The device includes a current sensor and a controller. The current sensor senses current provided to the motor. The controller determines whether the sensed current is less than a threshold current value. The controller determines a charge accumulation by subtracting the sensed current from a threshold current value and adding the difference to a previous charge accumulation. The controller decreases a fold-back factor when the sensed current is greater than the threshold current value and the determined charge accumulation is greater than a threshold charge accumulation. The controller decreases the current provided to the motor by applying the fold-back factor to the current provided to the motor. The controller determines whether the sensed current is less than the threshold current value for a predetermined period of time. The controller determines whether the determined charge accumulation is less than or equal to a second threshold charge accumulation. The controller increases the fold-back factor when the sensed current is less than the threshold current value for the predetermined period of time and the determined charge accumulation is less than or equal to the second threshold charge accumulation. The controller increases the current provided to the motor by applying the fold-back factor to the current provided to the motor.